The subject line is all an open rate proves
A high open rate means the subject line got someone to open the email. That is all it means. It says nothing about whether the content was relevant, whether the recipient did anything afterwards, or whether the send contributed to the business. The metric is shaky at its foundation, too: preloaded images and inboxes that shield the recipient count as opens without a human having read a line. You are measuring a gesture, not a value.
An opened send can cost more than an unopened one
A send with a high open rate and no follow-on action is a more expensive failure than one with a low open rate and a clear effect. Not because it cost more to send, but because it looks like a success. It passes through the report unchallenged, and nobody asks what actually happened. The weak send gets reviewed and reworked. The heavily opened one lives on unchanged, send after send, carrying the same hollow middle every time.
The channel is owned, and that raises the bar
An email goes to people who have given you permission to get in touch. That property is what separates the channel from bought reach, and the permission is not free to waste. Every email that gets opened and forgotten wears a little of it away. Relevance is decided before the send is even written, in the segmentation: who receives it, and what those particular people should get out of opening it. An email that fits everyone on the list equally badly may well be opened by plenty of people. It still builds nothing.
The open is a gesture. The business is decided by what the recipient does in the minute after it.
Measure the step after the open
Change the question you ask. Don't ask how many opened, but how many did something you actually wanted: read on, requested a quote, came back. Follow the recipient all the way to what happens on the page the email leads to, not just to the inbox. And treat the measurement as part of the rest of the content work rather than an island of its own: the channel is a way to distribute content, and content is judged on what it does to the recipient. The open rate can stay on as a diagnosis of the subject line. It just must never be the verdict on the send.